Removed undefined constant ScriptHandler::NEW_STRUCTURE_NOTIFIER

Fixed #138

The constant `ScriptHandler::NEW_STRUCTURE_NOTIFIER` was used on [`Resources/bin/build_bootstrap.php#L49`] (https://github.com/sensiolabs/SensioDistributionBundle/blob/master/Resources/bin/build_bootstrap.php#L49). This constant was temporarily created - then removed - during the development of the PR #118 by @romainneutron.